﻿Vol. 66.~] DEDOLOMITIZA.TION IN THE MARBLE OE PORT SHEPSTONE. 519 



(I) CaMg(C0 3 ) 2 4- A1 2 3 = MgO.Al 2 3 + CaC0 3 + C0 2 . 

 Dolomite. Alumina. Spinel. Calcite. Carbon 



dioxide. 



(II) 2CaMg(C0 3 ) 2 + Si0 2 = 2MgO.Si0 2 + 2CaC0 3 + 2C0 2 . 

 Dolomite. Silica. Forsterite. Calcite. Carbon 



dioxide. 



The formation of the phlogopite can be explained theoretically by 

 the following somewhat simplified equation, in which the presence 

 of alkalis and water is ignored : — 



(III) CaMg(C0 3 ) 2 + A1,0 3 + 2Si0 2 



Dolomite. Alumina. Silica. 



= MgO.Al 2 3 2Si0 2 + CaC0 3 4- C0 2 . 

 Phlogopite. Calcite. Carbon 



dioxide. 



The spinel-forsterite rock is a perfect example of dedolomitization. 

 In this case the magnesia of the dolomite combined with alumina to 

 form spinel, and with silica to form forsterite, the residual car- 

 bonate of lime re-crystallizing as calcite. Whence the alumina and 

 silica were derived in this particular case there is no evidence to 

 show, since no unaltered fragment of the foreign rock is present in 

 the collected material. The reaction which probably took place 

 assumes the addition of silica and alumina to the dolomite, and can 

 be represented by the following equation : — 



(IV) 3CaMg(C0 3 ) 2 +' A1 2 3 + Si0 2 



Dolomite. Alumina. Silica. 



= MgO.Al 2 3 4- 2MgO.Si0 2 4- 3CaC0 3 4 3C0 2 . 

 Spinel. Forsterite. Calcite. Carbon 



dioxide. 



The fact that neither periclase nor brucite is found in the normal 

 marble points to the metamorphism having been conducted under 

 such high-pressure conditions that the dolomite molecule was unable 

 to part with any of its carbon dioxide, thus differing from the con- 

 ditions under which the metamorphism of the Durness Limestone 

 took place, as described by Dr. J. J. H. Teall 1 and Mr. A. Harker. 2 

 In the latter instance, the dolomite, where free from silica, was con- 

 verted into an aggregate of calcite and brucite ; but where silica, in 

 the form of chert, was present, silicates of magnesia and lime, such as 

 forsterite, diopside, and tremolite, were formed. In the marble of 

 Port Shepstone, on the other hand, the normal dolomite crystallized 

 without decomposition, while the formation of silicates of magnesia, 

 lime, and alumina was restricted to areas in which a considerable 

 amount of foreign material was present. This foreign material 

 consisted, in many cases, of the granite and other blocks above 



1 Geol. Mag. 1903, p. 513 ; see also ' The Geological Structure of the North- 

 West Highlands of Scotland ' Mem. Geol. Surv. 1907, p. 453. 



- ' The Tertiary Igneous Rocks of Skye' Mem. Geol. Surv. 1904, p. 144. 



